The Chemistry of Life: Water

By the end of this study, students are expected to:
- State the meaning of a dipole.
- Explain why water forms a dipole.
- Explain hydrogen bonding using water as the primary example.
- Illustrate hydrogen bonding between water molecules in a diagrammatic format.
- Explain the mechanism by which water acts as a solvent.
- Describe three additional properties of water and their specific uses for living organisms.
The core learning objective is to understand the importance of water as a solvent in transport, specifically focusing on its dipole nature.

An atom becomes stable through the interaction of its electrons in two primary ways:
- Ionic Bonding: This involves the complete transfer of electrons from one atom to another.
- Covalent Bonding: This involves the sharing of electrons between atoms.

Ionic bonding is defined as the electrostatic attraction between oppositely charged ions.
During this process, atoms may either give or receive electrons to achieve stability.
The gain of electrons results in the formation of Anions, which are negatively charged ( $-ve$ ) ions.
The loss of electrons results in the formation of Cations, which are positively charged ( $+ve$ ) ions.

A covalent bond is a chemical link formed when two atoms share one or more pairs of electrons.
Molecular configurations vary based on electron sharing:
- Neutral Molecules: These occur when electrons are shared equally by the same types of atoms.
- Polarized Molecules: These occur when there is an uneven pairing of electrons between different types of atoms.

When electrons are not shared equally, they are more attracted to one atom than the other.
Electronegativity is defined as an atom's ability to attract shared electrons within a chemical bond.
Molecules become polarized because of differences in electronegativity.
The result is a polar molecule that ends up being slightly charged instead of neutral.

Water ( $H_2O$ ) exhibits covalent bonding characterized by uneven electron pairing.
Oxygen ( $O$ ) is significantly more electronegative than hydrogen ( $H$ ).
The oxygen end of the molecule becomes partially negative ( $\delta-$ ).
The hydrogen end of the molecule becomes partially positive ( $\delta+$ ).
This unequal distribution of charges makes water a polar molecule, meaning it contains a dipole.

Hydrogen bonding occurs when the slightly negative oxygen atom of one water molecule attracts the slightly positive hydrogen atom of a different water molecule.
Individual hydrogen bonds are relatively weak; they frequently break and reform.
However, when a large number of hydrogen bonds are present simultaneously, they form a strong and stable structure.

Because water is a polar solvent, many ionic substances can dissolve in it.
The solvation process involves the separation of ions:
- Cations (positive ions) are surrounded by the slightly negative oxygen atoms of the water molecules.
- Anions (negative ions) are surrounded by the slightly positive hydrogen ends of the water molecules.

Water also acts as a solvent for certain non-ionic substances, provided they contain polar groups.
Sugars, for instance, contain polar hydroxyl ( $OH$ ) groups.
Water can dissolve these substances because hydrogen bonds form between the water molecules and the slightly charged $OH$ groups of the sugar.

High Specific Heat Capacity is defined as the energy required to change the temperature of $1\,g$ of water by $1^{\circ}C$ .
Water absorbs heat slowly because the hydrogen bonds between the molecules require a significant amount of energy to separate.
Biological Importance:
- This property helps keep aquatic temperatures constant.
- It prevents sudden fluctuations in temperature within biological systems.

Latent Heat is the energy released or absorbed by a substance during a change in its physical state that occurs without a change in its temperature.
Latent Heat of Fusion: The energy required to change a substance from ice to liquid water.
Latent Heat of Vaporization: The energy required to vaporize water.
- This high latent heat of vaporization makes water an effective coolant for living organisms.

Water reaches its maximum density at $4^{\circ}C$ .
As water cools below this point toward freezing, the molecules become more widely spaced.
Consequently, ice is less dense than liquid water and floats.
Biological Importance: This causes bodies of water, such as ponds, to freeze from the top down. This insulation allows aquatic organisms to survive at the bottom during winter.

Cohesive Property: This is the attraction between water molecules themselves.
Adhesive Property: This is the attraction between water molecules and the molecules of different substances.
High Surface Tension:
- Water behaves like a stretched elastic membrane.
- Surface tension is defined as the force required to break the surface of water.
- The attraction between water molecules (cohesion) is greater than the attraction between water and the surrounding air.

Task 1: Draw the dipole structure of a water molecule, labeling the partial charges ( $\delta-$ and $\delta+$ ).
Task 2: Create a diagram illustrating the hydrogen bonding between multiple water molecules.
Task 3: Explain the process by which water dissolves Sodium Chloride ( $NaCl$ ) using diagrams to show the orientation of water molecules around the sodium and chloride ions.